As optical discs for recording/reproducing the information by irradiation of a laser beam, a magneto-optical disc, a variety of write-once optical discs, a digital audio discs, such as a so-called compact disc or an optical video disc, have been put to practical use.
Of these optical discs, the compact disc or the optical video disc is a play-only disc, and is comprised of a transparent substrate, in which data pits corresponding to information signals are formed as recesses, and a reflective layer formed on the transparent substrate.
As the transparent substrate, a disc-shaped substrate, formed by injection molding a resin, such as polycarbonate, is predominantly employed, since the cost may be lowered especially in case of mass production. On concentric or spirally extending track(s) of the transparent substrate, there are formed data pits as recesses. The reflective layer is layered on the surface of the transparent substrate carrying the data pits. In general, an Al reflective film is used because it has high reflectance and good thermal conductivity.
With the above-described play-only optical disc, the difference in the amount of reflected light between the pits and mirrors, that is disc portions devoid of the pits, on laser light irradiation from the transparent substrate, is detected, and a bit pattern on the track(s) is accordingly reproduced.
For correctly reproducing error-free signals by the above technique, laser light spots need to be radiated correctly on a track on which a bit pattern to be read out is formed. To this end, the optical disc driving apparatus performs tracking servo of the optical pickup device. Among optical disc systems for scanning the concentric or spirally extending track(s) with the laser light beam for recording/reproducing various sorts of data, there are known a CLV system for rotationally driving the optical disc at a constant linear velocity (CLV) for recording/reproducing data, and a CAV system for rotationally driving the optical disc at a constant angular velocity (CAV) for recording/reproducing data. There are also known a continuous servo system in which tracking control is done using a continuous pre-groove formed along the track, and a sample-servo system in which tracking control is done using servo areas provided discretely on the track(s).
A conventional optical pickup device for a magneto-optical disc is configured as shown for example in FIG. 1. During reproduction or recording, a laser beam as an outgoing light beam of a P-polarized component is radiated from a laser diode 1. This outgoing light is collimated by a collimator lens 2 so as to be shaped by a shaping prism 3 to fall on an S-polarizing beam splitter 4.
The S-polarizing beam splitter 4 has a light polarization beam splitter film 4a having characteristics of reflecting the S-polarized light component having the direction of polarization perpendicular to the P-polarized light component and reflecting and transmitting 50% of the P-polarized light and the remaining 50% thereof, respectively. Thus, one-half of the outgoing light of the P-polarized light, incident on the S-polarizing light beam splitter 4, is reflected, while the remaining one-half thereof is transmitted. The outgoing light transmitted through the S-polarizing beam splitter 4 is reflected by a 45.degree.-mirror 5 and thence radiated via an objective lens 6 on a magneto-optical disc 7.
During recording, data is supplied via an input terminal 8 on a magnetic head 9. This drives the magnetic head 9 responsive to the data for.generating a magnetic field corresponding to the data. This magnetic field is impressed on an area of the magneto-optical disc 7 irradiated with the laser beam for recording data thereon.
A reflected light beam is produced by the magneto-optical disc 7 being irradiated with the outgoing light. This reflected light is reflected by the 45.degree. mirror 5 via the objective lens 6 to fall on the S-polarizing beam splitter 4.
This reflected light is polarized responsive to data recorded on the magneto-optical disc 7 and thereby reflected as an S-polarized light component. The amount of the polarized light is delicate and the major portion of the reflected light is the P-polarized light component. The S-polarizing beam splitter 4 reflects 100% of the S-polarized light component, while reflecting and transmitting 50% and the remaining 50% of the P-polarized light component, respectively. Thus, as for the reflected light, the reflected portion of the S-polarized light beam is reflected in its entirety by the S-polarization beam splitter 4 to fall on a polarizing beam splitter 10, while one-half and the remaining half of the P-polarized light component are reflected by and transmitted through the S-polarizing beam splitter 4, respectively.
The polarizing light beam splitter 10 has a polarization light beam splitter film 10a has characteristics of transmitting the P-polarized light component in its entirety and reflecting the S-polarized light component in its entirety. Consequently, as for the reflected light incident on the polarizing beam splitter 10, the reflected light of the P-polarized component is transmitted through the polarization light beam splitter film 10a to fall on a servo signal detection system 11, while the light of the S-polarized component is reflected by the polarizing beam splitter film 10a so as to fall on a data detection system 12.
The reflected light of the P-polarized light component, incident on the servo signal detection system 11, is converged by a lens 13 and a cylindrical lens 14 to fall on a photodetector 15 used for detecting servo signals. The photodetector 15 receives the reflected light of the P-polarized light component and supplies a detection signal corresponding to the received light to a servo signal generating circuit, not shown. The servo signal generating circuit generates focusing error signals and tracking error signals, based upon the detection signal from the photodetector 15, and transmits the focusing error signal and the tracking error signal to servo control circuits, not shown. These servo control circuits effectuate tracking error control and focusing error control based upon the focusing error and tracking error signals. This assures data reproduction under just-track and just-focus conditions at all times. The S-polarization light beam splitter 4 reflects 50% of the P-polarized light component, while transmitting the remaining 50% thereof, while the servo signal detection system 11 detects the tracking error. and focusing error signals based upon the reflected light of the P-polarized light components reflected by the S-polarizing light beam splitter 4. Since the major portion of the reflected light is the P-polarized light component, the tracking and focusing errors may be detected with a sufficient light volume if the S-polarization light beam splitter 4 is configured for reflecting and transmitting 50% and the remaining 50% of the P-polarized light component, respectively.
The reflected light of the S-polarized light component, reflected by the polarization beam splitter 10, is converted by a .lambda./2 plate 16 of the data detection system 12 into a reflected light of the P-polarized light component which is then incident via a condensing lens 17 on a polarization beam splitter 18. The polarization beam splitter 18 has a polarization light beam splitter film 18a having characteristics of reflecting 50% of the P-polarized light component and transmitting the remaining 50% thereof. Thus the reflected light of the p-polarized light component, incident on the polarization beam splitter 18, is divided by the polarization beam splitter film 18a into two portions which are incident on data-detection photodiodes 19A, 19B.
The photodetectors 19A, 19B receive the two reflected light beams and transmit detection signals of signal levels corresponding to the volumes of the received light to a data detection circuit, not shown. The data detection circuit detects data based upon.the detection signals and transmits the detected data to a data processing system. The data processing system processes the data in a preset manner and transmits the processed data to an external equipment, such as a computer or a speaker.
There has also been known a phase-change type optical disc 24 for recording data by exploiting changes in structure between the amorphous sate and the crystal state of a substance.
The optical pickup device, reproducing data from the phase-change optical disc, has a structure as shown in FIG. 2, and is configured for radiating a laser beams as an outgoing light of, for example, the P-polarized light component. The outgoing light is collimated by a collimator lens 21 and reflected by a 45.degree. mirror 22 to fall on a hologram film 17.
The hologram film 27 is formed as a planar hologram in the shape of a refractive lattice functioning as a polarization beam splitter for transmitting the light of the P-polarized light components as it is and for radiating the light of the S-polarized light component after changing its light path. Thus the outgoing light of the P-polarized light component, incident on the hologram film 27, is directly transmitted through the hologram film 27 to fall on a quarter wave plate 28. The quarter wave plate 28 converts the linear-polarized radiation into circular polarized light which is radiated via an objective lens 29 on the phase-change optical disc 24.
The circular-polarized outgoing light is radiated on and reflected by the phase change optical disc 24, whereby the circular-polarized reflected light, opposite in the direction of polarization to the outgoing light, is produced. This S-polarized reflected light falls via the objective lens 29 on the quarter wave plate 28. When the circular-polarized light, opposite in the direction of polarization to the circular-polarized laser light beam, is incident on the quarter wave plate 28, the quarter wave plate 28 converts it into a reflected light of the S-polarized light component. This reflected light of the S-polarized light component falls on the hologram film 27.
The hologram film 27 has characteristics of functioning as a polarizing beam splitter for bending the light path of an incident light of the S-polarized light component a pre-set angle and radiating the light along the bent optical path. Thus the reflected light of the S-polarized light component, incident on the hologram film 27, has its light path bent by a pre-set angle by the hologram film 27, so as to be radiated on two photodetectors 26a, 26b of the laser module 20 via a 45.degree. mirror 22 and the collimator lens 21.
The photodetectors 26a, 26b receive the reflected light and output detection signals corresponding to the received light volumes. These detection signals are supplied to signal processing systems, not shown. These signal processing systems detect the focusing and tracking error signals and data recorded on the phase change optical disc 24, based upon detection signals from the photodetectors 26a and 26b, and transmit these to a servo control system and to a data processing system. This enables the data to be read under the just-track and just-focus states.
The optical pickup device for the phase-change optical disc, shown in FIG. 2, is configured for bending the light path of the reflected light by exploiting the characteristics of the hologram film 27. Thus the reflected light can be received by the photodetectors 26a, 26b provided in the vicinity of the laser diode 25; thus enabling the overall light path to be reduced. In addition, the optical pickup device itself and the apparatus provided with such optical pickup device can be reduced in size.
However, with the optical pickup device for the magneto-optical disc, in which the outgoing light and the reflected light are split using the polarizing light beam splitters 4, 10 and 18 for detecting the data and the focusing and tracking error signals, the number of component parts and hence the costs are increased. In addition, since it is necessary to secure light paths for the reflected lights split by the beam splitters 4, 10 and 18, the optical pickup device itself is increased in size.
On the other hand, the P-polarized light components are reflected and transmitted in amounts of 50% by the polarizing light beam splitter 10. This reflectance is set on the basis of the volume of light radiated on the servo signal detection system 11 and the shot noise of the photodetector 15 for servo signal detection or the noise produced by double refraction by the magneto-optical disc 7. The coupling efficiency and the S/N ratio are related to each other by trade-off, such that, if the coupling efficiency is increased, the S/N ratio is lowered, whereas, if the S/N ratio is improved, the coupling efficiency is deteriorated.
Although the light path length may be reduced with the optical pickup device for the phase change optical disc for reducing the size of the optical pickup device itself, it is difficult to detect the playback data of low signal intensity from the magneto-optical disc because the quarter wave plate 28 is employed.
It is therefore an object of the present invention to provide an optical pickup device and an optical disc driving apparatus whereby the polarizing beam splitter is eliminated and the number of components may be diminished for reducing the size and cost.
It is another object of the present invention to provide an optical pickup device and an optical disc driving apparatus whereby the coupling efficiency and the S/N ratio may be improved for satisfactorily reproducing data recorded on the magneto-optical disc.